As is known in the art, the rapid growth of Internet of Things (IoT) devices has led to a corresponding growth in the adoption of near-field wireless charging for various applications. However, as the number of wireless power receivers grows, so will the number of wireless charging circuits and devices (collectively “chargers”) that might be not strictly standards compliant or not intended or appropriate for use with a particular device (collectively “counterfeit” chargers). Given the critical nature of at least some tasks performed by IoT devices, protecting IoT devices from harsh transient signals (or more simply “transients”) imposed by counterfeit wireless chargers is crucial for at least several reasons not including the need to avoid damage to the devices due to such harsh transients. These transients could have potentially destructive impacts on both the receiver's electronics and the battery being charged. This further raises the challenge that underdamped LC resonant tank circuits (or more simply “tanks”) used by at least some resonant wireless power transmission (WPT) systems tend to cause overvoltage or overcurrent conditions in response to the transients imposed by counterfeit chargers.
Secure hash algorithm (SHA)-based cryptographic authentication protocols have been implemented commercially for the purpose of avoiding a device using a potentially damaging charger (i.e. a counterfeit wireless charger). These solutions use a receiver-based cryptographic element in the device to be charged that generates a so-called “challenge” (i.e. a request for authentication) using a predetermined key. A charger having an appropriate key can then decrypt and respond to that challenge. The receiver in the device to be charged is open circuited until it receives the correct response, upon which the device to be charged begins drawing energy from the charger.
While a similar challenge-response protocol for charger authentication could be employed for incorporating secure charging into WPT, this protocol is only well-suited to a one-charger, one-receiver scenario. The projected scale of IoT wireless power receivers in the near future would require multiple receivers with multiple chargers, in such scenarios authentication based upon a pre-shared secret (symmetric key) would be unsustainable and unscalable. Symmetric key authentication between the receiver and the charger requires that the receiver either be pre-programmed with the private keys of all possible chargers or be capable of exchanging a new key upon encountering a new charger. Such an authentication process requires all chargers and receivers share a master key that facilitates the key exchange over the same communication channel, thus introducing a weak point into the system.
Conversely, public key authentication uses two separate keys—a publicly known key used by the receiver for generating the challenge (public key) and its associated private key that is known only to the charger and is used for generating the response. The distribution of the charger public keys can be handled by issuing certificates signed by a trusted certificate authority, in a way similar to the key-exchange handshake implemented in the transport layer security (TLS) protocol.
In a scenario where, multiple receivers are coupled to the same charger, the power delivered to a receiver is a strong function of its proximity and orientation (which is related to the magnetic coupling coefficient) with respect to the charger coil, with more power going to the closer receiver. This physically imposed constraint might not necessarily reflect the actual energy requirements of the various receivers.